1. Technical Field
The present invention relates to an apparatus for estimating the charged state of a battery mounted on a vehicle, and in particular, to such an estimating apparatus in which the battery powers not only starting means that gives initial rotation to the output shaft of an internal combustion engine mounted on the vehicle but also control means for idle stop and automatic start of the engine.
2. Related Art
Recent vehicles are often provided with an apparatus for estimating charged state of an on-vehicle battery. This kind of apparatus is exemplified by Japanese Patent Laid-open Publication No. 2006-258070. This publication discloses an apparatus, in which the charged state of an on-vehicle battery is estimated when the vehicle is stopped, and it is determined whether or not the battery is able to output a necessary power when the engine (internal combustion engine) is started. When it is determined that the necessary power can be outputted, idle stop control is allowed to automatically stop the engine. Hence, depending on whether or not restarting the engine is possible, the idle stop control can be allowed or prohibited.
It is usual that the on-vehicle battery also functions as power supply means for the engine. The battery needs to have a minimum voltage which is necessary for securing the reliability of operations of a controller mounted on the vehicle. Even though it is possible to power a starter that gives initial rotation to the output shaft of the engine in restarting the engine, a large drop in the battery voltage may occur in restarting the engine. In this case, an excessive drop in the battery voltage may lead to degraded reliability of operations of the controller. Therefore, when the vehicle is equipped with an automatic stop/start apparatus, it is desired that the minimum battery voltage in starting the engine be estimated for securing reliability of operations of the controller.
The minimum value of the battery voltage generated when the engine is started can be estimated on a formula of “ΔImax·R+V”, wherein R denotes the current internal resistance, ΔImax denotes the maximum discharge current amount of a battery in starting the engine, and V denotes the current voltage of the battery.
The maximum discharge current amount is an amount of discharged current which flows through a starter so as to sharply rise immediately after powering the starter. Then this discharged current reduces rapidly as the starter starts its rotation. The maximum discharge current amount ΔImax changes depending on various factors including the battery temperature and/or the discharge capacity of the battery. Accordingly, giving a fixed value to this maximum discharge current amount ΔImax will invite deterioration in the estimation accuracy.
Further, it is also difficult to accurately detect the maximum discharge current amount ΔImax by a sensor. That is, the maximum discharge current amount ΔImax will have a very large value which is hardly caused in cases other than starting the engine. A current sensor which is also able to detect the discharge current is very high in parts cost. In this regard, it is not desired to install such a high-cost current sensor for measurement in only starting the engine.